Hitherto, a silicone rubber sponge is well known as a material excellent in the weather resistance, the electric characteristics, and the compression set.
The silicone rubber sponge is fundamentally obtained by compounding a thermosetting silicone rubber composition with an organic foaming agent and a curing agent and foaming and curing the compounded mixture by heating, but in this case, it is important that the silicone rubber sponge obtained is excellent in the surface smoothness of the skin layer, has no surface tackiness, has a good compression set, and does not impair the inherent properties that a silicone rubber possesses. From such a view point, hitherto an organic peroxide or an addition reaction type curing agent has been generally used as a curing agent, and azobisisobutyronitrile (AIBN) has been generally used as a foaming agent.
However, when azobisisobutyronitrile (AIBN) is used, a post cure of a long time is required in order to remove a decomposition product which is considered to be harmful, which causes a problem of prolonging the production time. Hence for shortening the production time, shortening the post cure time has been desired.
For the purpose, methods of using other foaming agents than azobisisobutyronitrile have been investigated, and for example, a method of using azodicarbonamide (ADCA) or dinitropentamethylenetetramine (DPT) is proposed as described in JP-A-55-29565 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, these foaming agents scarcely foam by simply compounding with a silicone rubber compound containing organic peroxides and/or addition reaction curing agents and undergo free foaming only under a hot blast, like the case of using azobisisobutyronitrile. Also in the case of using these foaming agents, there are problems that the silicone rubber sponges formed are greatly poor in the surface smoothness and give a tacky feeling on the surfaces thereof.
Recently, as a method of solving these problems, a method of using a silicone polymer, wherein a 1-ethyl-1-butyl group, an ethylidenenorbonyl group, etc., is used as the crosslinking group in place of a conventional vinyl group, as the base polymer (JP-A-2-16132) and a method of using a silicone polymer, wherein a cycloalkyl group such as cyclohexyl group, etc., is used as the crosslinking group, as the base polymer (JP-A-2-251542) are proposed.
However, although the foaming property, the surface smoothness and the surface tackiness are improved in these methods, these methods have a defect that the silicone rubber sponges formed are greatly poor in the compression set characteristics as compared with the sponge moldings obtained using azobisisobutyronitrile as the foaming agent.
Furthermore, the case of using a silicone polymer which does not have an aliphatic unsaturated group such as a vinyl group, etc., that is, a silicone polymer wherein the organic groups bonded to the silicon atom are composed of a methyl group, an ethyl group, a propyl group, a phenyl group, a 3,3,3-trifluoropropyl group, etc., as the base polymer is reported. However, in this case, although the problems of the surface smoothness and the surface tackiness are solved, it is very difficult to control crosslinking and also the compression set characteristics are greatly poor.
On the other hand, the prior art does not show that 1,1'-azo-bis(1-acetoxy-1-phenylethane) is compounded with a silicone rubber composition, but U.S. Pat. Nos. 4,129,531 and 4,460,748 disclose the embodiment that 1,1'-azo-bis(1-acetoxy-1-phenylethane) is compounded with polymers or copolymers. Those U.S. patents achieve curing or simultaneous curing/foaming of polymers or copolymers by using a specific azo compound as a curing agent.
However, such an azo compound is not recently used at all as the curing agent for the thermosetting silicone rubber composition. The reason for this is that the vulcanization effect of the azo compound to the thermosetting silicone rubber composition is insufficient as compared with organic peroxides or addition reaction type curing agents, and a practical rubber elastomer cannot be obtained.
In order to confirm this fact, the present inventors conducted experiments to examine the effect of 1,1'-azo-bis(1-acetoxy-1-phenylethane) on the thermosetting silicone rubber composition. As a result, only very insufficient vulcanization effect was obtained, and a practical rubber elastomer could not be obtained.
In this regard, U.S. Pat. No. 2,613,119 discloses that an aliphatic azo compound functions as a useful vulcanizing agent for a silicone rubber. However, on the other hand, JP-B-44-461 discloses that the vulcanization effect of azobisisobutyronitrile (AIBN) which is the representative example of the aliphatic azo compounds is insufficient.
U.S. Pat. No. 4,129,531 discloses a method for crosslinking polymers using azo esters including 1,1'-azo-bis(1-acetoxy-1-phenylethane). However, a silicone rubber is not disclosed in the polymers. Further, the object thereof is to crosslink an elastomer other than the silicone rubber.
U.S. Pat. No. 4,460,748 discloses the effect as a curing agent of azo compounds exemplifying 1,1'-azo-bis(1-acetoxy-1-phenylethane) similar to U.S. Pat. No. 4,129,531. Although examples of the polymers or copolymers disclosed therein include a silicone rubber, a combination of 1,1'-azo-bis(1-acetoxy-1-phenylethane) and the thermosetting silicone rubber composition is not disclosed, and there is no disclosure to suggest such a combination. Basically, U.S. Pat. No. 4,460,748 uses a crosslinking intensifier as the essential component, which is not used in the present invention. If such a crosslinking intensifier is added to the composition of the present invention, compression set or heat resistance of the sponge may greatly deteriorate, or tackiness may occur on the sponge surface.
The object of U.S. Pat. No. 4,460,748 is to enable polymers or copolymers to cure by adding a crosslinking intensifier even if the amount of the azo compound added is small. However, the thermosetting silicone rubber composition does not substantially cure even if a large amount of 1,1'-azo-bis(1-acetoxy-1-phenylethane) is added as a curing agent. Therefore, it is meaningless to add the crosslinking intensifier.
As described above, to obtain a silicone rubber sponge having a high foaming ratio and a uniform cell structure, a method of using an organic peroxide or an addition reaction type catalyst as the curing agent and also using azobisisobutyronitrile as the foaming agent has hitherto been generally used, but in this case a post cure of long time is required in order to remove a decompostion product which is considered to be harmful. Thus, there is a problem in the productivity as described above, and the development of a technique substituting for the above-described method has been demanded.
However, the conventional methods proposed or reported as described above have problems in the foaming property, the surface smoothness, and the surface tackiness as well as there is a defect that the compression set characteristics are greatly poor. Further, where an azo compound is used as a curing agent, there is the problem that the vulcanization effect to the thermosetting silicone rubber composition is insufficient, and a practical rubber elastomer cannot be obtained. Thus, satisfactory methods of obtaining silicone rubber sponges have not yet been obtained in the actual situation.